Termination of transcription is a control point that is frequently utilized in prokaryotes to regulate gene expression. However, the extent to which this is true in eukaryotes is not yet clear. At present, the termination system which is best understood in eukaryotes is that of RNA polymerase I, due largely to studies on the frog, Xenopus laevis, in our own laboratory and studies on the mouse in the laboratory of Ingrid Grummt. The sites of termination are known, various mutants of these sites are available, and good assay systems for termination have been developed both in vivo and vitro. Fractionation of the proteins involved in termination is underway and we have begun to propose testable models for the process. In this application we propose to continue studies of RNA polymerase I termination in Xenopus. The proteins involved in termination will be identified and purified using and in vitro termination system. The genes for these proteins will be cloned and characterized. The heart of the proposal then is to determine the molecular mechanism by which these components effect termination of transcription by RNA polymerase I.